User account and security

On Vista, in order for a system to see another machine's share, it asks for
the user's credentials. I know this can be tuned off using Password
protection for shares. However, regardless, if both the systems have the same
user account with the same password (not necessarily logged in as that user
but just if they have it), then things become very simple and the other user
can see and open the shares.

So here is the question: I am thinking of automatically creating a user
account with a GUID like password on these systems using standard Win32
API's. However, I am afraid that by creating user accounts automatically
(albeit with a GUID like password), I maybe opening up the user system
inadvertently to some security risk beyond what I am trying to achieve.

Does anyone have an opinion on this or see any major issues in doing this?

Thanks.